It is a deadline that lies at the heart of one of the most controversial foreign policy decisions that President Obama has made. His announced timeline for withdrawal from Afghanistan has drawn fire from many quarters. Sen. John McCain (R-Ariz.) and others have argued that by making U.S. plans clear, the president is sending the wrong message to the Taliban and complicating efforts to defeat it. Announcing "dates certain" for withdrawal, according to this view, is political, not strategic.

Nothing could be further from the truth. There is a distinct strategic logic behind the announcement of withdrawal dates.

The difference between the McCain and Obama positions lies in the interpretation of what the main objective is in Afghanistan: Is the United States trying to defeat the Taliban, or to establish a viable national government? McCain clearly thinks it is the former. He is correct that it is a strange strategic decision to communicate our plans to the enemy, if indeed the Taliban was our primary concern.

But the most important factor in Afghanistan — or what strategists sometimes refer to as the "center of gravity" — is not the Taliban but the Karzai government. It is not our enemies who represent the main obstacle to success but our allies.

The Afghan security forces have nearly 350,000 trained men under arms, compared with perhaps one-tenth that number of insurgents. Polling indicates that the people have no love for the Taliban and do not wish to see it return. In other words, the insurgents can kill many people and make life miserable, but they won't bring down the Karzai government. Only Karzai can do that, through mismanagement, endemic corruption and incompetence.

As it stands, Karzai has little incentive to take steps toward better governance. As long as he can count on the support of NATO troops, there is no reason to cooperate with the regional warlords, improve services for his people or try to cut deals with local Pashtun leaders who might be seduced away from the Taliban. He has to make no tough decisions as long as his basic security is assured.

Without a sense of urgency on Karzai's part, the insurgency in Afghanistan is unlikely to go away any time soon. It certainly cannot be permanently defeated as long as it has safe haven across the border in Pakistan. The goal of the United States is not to force the various Taliban groups to surrender but to encourage the ineffective, venal Karzai government to make the kind of adjustments that would allow it to survive on its own. And a deadline is the only way to accomplish that mission.

The Obama administration is thus in an awkward position. It cannot fully explain the strategic logic of its policy without insulting its partners in an undiplomatic, counterproductive fashion. Setting deadlines for withdrawal acknowledges that reality.

It is worth remembering what happened when the Soviet Union attempted to build a nation in Afghanistan. Once Soviet leader Mikhail Gorbachev announced in April 1988 that Soviet troops would leave by February, their puppet in Afghanistan, Najibullah, was forced to get serious about governing without friendly foreign troops.

The deadline worked. The Soviets withdrew according to schedule, by which time Najibullah's government had consolidated its position enough to hold on to power far longer than the CIA thought it would. As it turned out, that government outlasted its sponsor's, and collapsed only after the aid spigot from Moscow went dry.

The lesson is this: Although Najibullah was even less popular than Karzai is now and had much stronger opposition, once faced with the certainty of having to stand on his own, he got his act together. Since Western aid will continue after the troops are gone, the post-NATO Afghan government would probably be able to persevere indefinitely.

The Karzai government will never be ready to stand on its own unless it is given incentive to do so. Setting a deadline for withdrawal is the correct strategic choice.

Christopher J. Fettweis, an associate professor of political science at Tulane University, previously taught strategy as an assistant professor of national security affairs at the U.S. Naval War College.